I agree with @casportpony 's advice. If it was only the ducklings showing symptoms .... probably just shock from cold water.
Though if you're concerned about the toxicity of your water, and you're waiting on results from a water test, I'd ... probably also hold off on the pool. But for future...
Good to hear! Maybe hold off on a pool for a bit? Maybe just provide a shallow dish filled with the bottled water for them to wash with.
How old are they?
Put a case of bottled water out in the sun to warm it & use that as your drinking water? I've also seen ducklings get 'shocked' by cold water, so it's always best to warm the water beforehand if at all possible.